U.S. patent number 6,977,563 [Application Number 10/670,234] was granted by the patent office on 2005-12-20 for thin-film piezoelectric resonator and method for fabricating the same.
This patent grant is currently assigned to TDK Corporation. Invention is credited to Katsuhiko Gunji, Masaaki Imura, Eiju Komuro, Joseph Edward Albert Southin, Taku Takeishi, Roger William Whatmore.
United States Patent |
6,977,563 |
Komuro , et al. |
December 20, 2005 |
Thin-film piezoelectric resonator and method for fabricating the
same
Abstract
A thin-film piezoelectric resonator including a piezoelectric
thin film having piezoelectric characteristic, and an upper
electrode and a lower electrode arranged on opposite surfaces of
the piezoelectric thin film for applying an excitation voltage to
the piezoelectric thin film, wherein: each of the upper electrode
and the lower electrode includes a resonant portion, and a lead-out
portion; and the electrode thickness of at least one part of the
lead-out portion in at least one of the upper electrode and the
lower electrode is larger than the electrode thickness of the
resonant portion formed to be continued from the lead-out
portion.
Inventors: |
Komuro; Eiju (Tokyo,
JP), Imura; Masaaki (Tokyo, JP), Gunji;
Katsuhiko (Tokyo, JP), Takeishi; Taku (Tokyo,
JP), Whatmore; Roger William (Milton Keynes,
GB), Southin; Joseph Edward Albert (Bedfordshire,
GB) |
Assignee: |
TDK Corporation (Tokyo,
JP)
|
Family
ID: |
31987118 |
Appl.
No.: |
10/670,234 |
Filed: |
September 26, 2003 |
Foreign Application Priority Data
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Sep 27, 2002 [JP] |
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2002-282727 |
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Current U.S.
Class: |
333/187;
333/189 |
Current CPC
Class: |
H03H
3/04 (20130101); H03H 9/02094 (20130101); H03H
9/02149 (20130101); H03H 9/173 (20130101); H03H
9/174 (20130101); H03H 9/564 (20130101); H03H
2003/0428 (20130101) |
Current International
Class: |
H03H 009/00 () |
Field of
Search: |
;333/187,188,189,191
;310/312,321 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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60-189307 |
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Sep 1985 |
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JP |
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2000-278078 |
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Oct 2000 |
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JP |
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WO 99/37023 |
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Jul 1999 |
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WO |
|
Other References
Kiyoshi Nakamura, et al., "Thin Film Resonators and Filters",
International Symposium on Acoustic Wave Devices for Future Mobile
Communication Systems, pp. 93-99, Mar. 5, 2001. .
Su Q.X, et al., Edge Supported ZnO Thin Film Bulk Acoustic Wave
Resonators and Filter Design, Frequency Control Symposium and
Exhibition, 2000 Proceedings of the 2000 IEEE/EIA International
Jun. 7-9, 2000, pp. 434-440..
|
Primary Examiner: Takaoka; Dean
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. A thin-film piezoelectric resonator comprising: a piezoelectric
thin film having piezoelectric characteristic; and an upper
electrode and a lower electrode arranged on opposite surfaces of
said piezoelectric thin film for applying an excitation voltage to
said piezoelectric thin film, wherein each of said upper electrode
and said lower electrode includes a resonant portion and a lead-out
portion, and an electrode thickness of at least one part of said
lead-out portion in at least one of said upper electrode and said
lower electrode is larger than an electrode thickness of said
resonant portion formed to be continued from said lead-out
portion.
2. A thin-film piezoelectric resonator according to claim 1,
wherein said piezoelectric thin film has a thickness of not larger
than 5 .mu.m.
3. A thin-film piezoelectric resonator comprising: a piezoelectric
thin film having piezoelectric characteristic; and an upper
electrode and a lower electrode arranged on opposite surfaces of
said piezoelectric thin film for applying an excitation voltage to
said piezoelectric thin film, wherein each of said upper electrode
and said lower electrode includes a resonant portion and a lead-out
portion, and said lead-out portion in at least one of said upper
electrode and said lower electrode is different in electrode
material from said resonant portion formed to be continued from
said lead-out portion.
4. A thin-film piezoelectric resonator according to claim 3,
wherein at least one part of said lead-out portion is formed by
stacking layers with different electrode materials, where one of
said stacked electrode is formed to be continued from said resonant
portion.
5. A method of fabricating a thin-film piezoelectric resonator
including a piezoelectric thin film having piezoelectric
characteristic, and an upper electrode and a lower electrode
arranged on opposite surfaces of said piezoelectric thin film for
applying an excitation voltage to said piezoelectric thin film,
said method comprising the step of: forming said lower electrode
and said upper electrode, at least one of forming step of said
upper electrode and said lower electrode including at least two
thin film-forming and patterning processes, wherein a mask used in
the first patterning process is different in shape from a mask used
in the second patterning process or in the patterning process after
the second patterning process, said forming producing for at least
one of the upper electrode and the lower electrode a resonant
region and a lead-out portion extending from the resonant portion,
said lead-out portion having an electrode thickness different from
that of the resonant portion.
6. A filter including a thin-film piezoelectric resonator
comprising: a piezoelectric thin film having piezoelectric
characteristic; and an upper electrode and a lower electrode
arranged on opposite surfaces of said piezoelectric thin film for
applying an excitation voltage to said piezoelectric thin film,
wherein each of said upper electrode and said lower electrode
includes a resonant portion and a lead-out portion, and an
electrode thickness of at least one part of said lead-out portion
in at least one of said upper electrode and said lower electrode is
larger than an electrode thickness of said resonant portion formed
to be continued from said lead-out portion.
7. A duplexer including a thin-film piezoelectric resonator
comprising: a piezoelectric thin film having piezoelectric
characteristic; and an upper electrode and a lower electrode
arranged on opposite surfaces of said piezoelectric thin film for
applying an excitation voltage to said piezoelectric thin film,
wherein each of said upper electrode and said lower electrode
includes a resonant portion and a lead-out portion, and an
electrode thickness of at least one part of said lead-out portion
in at least one of said upper electrode and said lower electrode is
larger than an electrode thickness of said resonant portion formed
to be continued from said lead-out portion.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a thin-film piezoelectric
resonator, a filter and a duplexer including such piezoelectric
resonators, and a method for producing the thin-film piezoelectric
resonator.
In mobile communication apparatuses such as cellular phones which
have come into wide use recently, both reduction in size and
heightening in operating frequency have advanced year by year.
Therefore, both reduction in size and heightening in operating
frequency have been required in electronic parts used in such
mobile communication apparatuses.
Some mobile communication apparatuses have a duplexer for
performing switching between a transmit signal path and a receive
signal path in order to use one antenna for both transmission and
reception. The duplexer has a transmit filter capable of passing a
transmit signal but cutting off a receive signal, and a receive
filter capable of passing a receive signal but cutting off a
transmit signal.
Surface acoustic wave filters have been used as the filters in the
duplexer recently. Such a surface acoustic wave filter has features
that the filter can support frequencies up to 2 GHz, and that the
size of the filter can be reduced as compared with a ceramic
filter. In the present circumstances, however, there still remain a
lot of technical problems in order to apply the surface acoustic
wave filter to frequencies of not lower than 2 GHz at which the
mobile communication apparatuses will work in the future.
Therefore, a device called thin film bulk acoustic resonator
(hereinafter referred to as FBAR) has attracted public attention
recently, for example, as described in JP-A-2000-278078. The FBAR
is a thin-film piezoelectric resonator using resonance in a
direction of the thickness of a piezoelectric thin film. In the
FBAR, the resonant frequency can be changed according to change in
thickness of the piezoelectric thin film. It is conceived that the
FBAR can support frequencies of the order of several GHz.
As shown in FIGS. 10A and 10B, the related-art thin-film
piezoelectric resonator has a so-called coplanar structure which
includes a substrate 1, upper and lower barrier layers 2 and 3
formed on upper and lower surfaces of the substrate 1 respectively,
a lower electrode 4 formed on the upper barrier layer 2, a
piezoelectric thin film 5 formed on the lower electrode 4, and an
upper electrode 6 and ground electrodes 7 formed on the
piezoelectric thin film 5. Incidentally, the substrate 1 includes a
vibration space S.
K. Nakamura and H. Kobayashi "Thin Film Resonators and Filters",
International Symposium on Acoustic Wave Devices for Future Mobile
Communication Systems, Mar. 5, 2001, pp. 93-99 proposes another
type of the thin-film piezoelectric resonator. In that, an acoustic
multi-layer film is formed without provision of any vibration space
in the substrate. That is, this type of the thin-film piezoelectric
resonator includes a substrate, an acoustic multi-layer film having
a plurality of layers disposed on the substrate, a lower electrode
disposed on the acoustic multi-layer film, a piezoelectric thin
film disposed on the lower electrode, and an upper electrode
disposed on the piezoelectric thin film. For example, the acoustic
multi-layer film is formed by laminating layers of high acoustic
impedance material such as aluminum nitride and layers of low
acoustic impedance material such as silicon oxide.
In a further type of the thin-film piezoelectric resonator, a
cavity is provided between the substrate and the lower electrode
(JP-A-60-189307). That is, this type of the thin-film piezoelectric
resonator includes a substrate, a lower electrode formed on the
substrate so as to form a cavity between the substrate and the
lower electrode, a piezoelectric thin film disposed on the lower
electrode, and an upper electrode disposed on the piezoelectric
thin film.
In each of these types of the thin-film piezoelectric resonators, a
metal such as Al, Pt, Au, Ag, Cr, Cu, Ti, etc. can be used as an
electrode material for forming the upper and lower electrodes which
are provided on opposite surfaces of the piezoelectric thin film
respectively.
Each electrode has a portion concerning a resonant portion, and a
lead-out portion. The resonant portion is defined as a portion
where the piezoelectric thin film is sandwiched between the lower
electrode and the upper electrode, and the lead-out portion is
defined as a portion except for the resonant portion. Generally,
these portions are integrally formed by a thin-film forming method.
Because the thickness of the resonant portion inclusive of the
electrode portion is decided on the basis of a required frequency,
the total thickness of the electrode inclusive of the lead-out
portion is decided necessarily.
Further, international publication WO99/37023 discloses a film bulk
acoustic wave device, comprising: a substrate; a bottom electrode
formed on one surface of the substrate; a piezoelectric film formed
on the bottom electrode; and a first top electrode formed on the
piezoelectric film, further comprises a second top electrode having
a larger mass load than the first top electrode, and formed on the
first top electrode on the piezoelectric film when viewed from the
center of the first top electrode, wherein the piezoelectric film
has a high-band-cut-off-type dispersion characteristic. The cut-off
frequency of a second top electrode portion piezoelectric film
having a large mass load can be lower than the cut-off frequency of
a first top electrode portion piezoelectric film, to thereby trap
the energy of the acoustic wave in a region of the first top
electrode portion side, so that good performance may be
feasible
The thickness of each electrode is limited by the resonant
frequency. For this reason, when each electrode is too thin, there
is a possibility that electric loss may occur. Incidentally, the
resonant characteristic of the thin-film piezoelectric resonator is
obtained by using a thickness vibration mode of the piezoelectric
thin film. Ripples due to a thickness shear mode may occur on a
pass signal in the passband according to the electrode material and
structure of the resonant portion.
SUMMARY OF THE INVENTION
Therefore, a first feature of the invention is to provide a
thin-film piezoelectric resonator, a filter and a dupluxer with a
insertion loss, and a method for fabricating the thin-film
piezoelectric resonator, the filter and the dupluxer.
A second feature of the invention is to provide a thin-film
piezoelectric resonator, a filter and a dupluxer with small
ripples, and a method for fabricating the thin-film piezoelectric
resonator, the filter and the dupluxer.
According to first aspect of the invention, it is provided a
thin-film piezoelectric resonator including a piezoelectric thin
film having piezoelectric characteristic, and an upper electrode
and a lower electrode arranged on opposite surfaces of the
piezoelectric thin film for applying an excitation voltage to the
piezoelectric thin film, wherein: each of the upper electrode and
the lower electrode includes a resonant portion, and a lead-out
portion; and the electrode thickness of at least one part of the
lead-out portion in at least one of the upper electrode and the
lower electrode is larger than the electrode thickness of the
resonant portion formed to be continued from the lead-out
portion.
According to second aspect of the invention, it is provided a
thin-film piezoelectric resonator including a piezoelectric thin
film having piezoelectric characteristic, an upper electrode and a
lower electrode arranged on opposite surfaces of the piezoelectric
thin film for applying an excitation voltage to the piezoelectric
thin film, and ground electrodes arranged on the same plane with at
least one of the upper electrode and the lower electrode, wherein:
each of the upper electrode and the lower electrode includes a
resonant portion, and a lead-out portion; and the electrode
thickness of at least one part of each of the ground electrodes is
larger than the electrode thickness of the resonant portion in the
upper electrode or the lower electrode formed on the same plane
with the ground electrodes.
According to third aspect of the invention, it is provided a
thin-film piezoelectric resonator including a piezoelectric thin
film having piezoelectric characteristic, and an upper electrode
and a lower electrode arranged on opposite surfaces of the
piezoelectric thin film for applying an excitation voltage to the
piezoelectric thin film, wherein: each of the upper electrode and
the lower electrode includes a resonant portion, and a lead-out
portion; and the lead-out portion in at least one of the upper
electrode and the lower electrode is different in electrode
material from the resonant portion formed to be continued from the
lead-out portion.
Preferably, in the thin-film piezoelectric resonator as discussed
above, at least one part of the lead-out portion is formed by
stacking layers with different electrode materials, where one of
the stacked layers is continued from the resonant portion.
Preferably, in the thin-film piezoelectric resonator as discussed
above, the piezoelectric thin film has a thickness of not larger
than 5 .mu.m.
It is provided a filter having thin-film piezoelectric resonators
as discussed above.
It is provided a duplexer having thin-film piezoelectric resonators
as discussed above.
According to fourth aspect of the invention, it is provided a
method of fabricating a thin-film piezoelectric resonator including
a piezoelectric thin film having piezoelectric characteristic, and
an upper electrode and a lower electrode arranged on opposite
surfaces of the piezoelectric thin film for applying an excitation
voltage to the piezoelectric thin film, the method including the
step of forming the upper electrode and the lower electrode, at
least one of the forming steps of the upper electrode and the lower
electrode including at least two film-forming and patterning
processes, wherein a mask used in the first patterning process is
different in shape from a mask used in the second patterning
process or in the patterning process after the second patterning
process.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a plan view showing a thin-film piezoelectric resonator
according to a first embodiment of the invention;
FIG. 1B is a sectional view taken along the line A-A' in FIG. 1A;
and
FIG. 1C is a sectional view taken along the line B-B' in FIG.
1A.
FIG. 2A is a plan view showing a filter according to a second
embodiment of the invention; and
FIG. 2B is a sectional view taken along the line A-A' in FIG.
2A.
FIG. 3 is a circuit diagram showing the circuit configuration of
the filter according to the second embodiment of the invention.
FIG. 4 is a view showing the shapes of masks used in a method for
fabricating the filter according to the second embodiment of the
invention.
FIG. 5A is a plan view showing a thin-film piezoelectric resonator
according to a third embodiment of the invention; and
FIG. 5B is a sectional view taken along the line A-A' in FIG.
5A.
FIGS. 6A to 6C are graphs showing impedance-frequency
characteristics in the case where the electrode material of each
resonant portion in the third embodiment of the invention is
changed, FIG. 6A showing the case where Al is used, FIG. 6B showing
the case where Au is used, FIG. 6C showing the case where Ag is
used.
FIG. 7 is a view showing the shapes of masks used in a method for
fabricating the thin-film piezoelectric resonator according to the
third embodiment of the invention.
FIG. 8 is a schematic sectional view showing a thin-film
piezoelectric resonator according to a fourth embodiment of the
invention.
FIG. 9 is a schematic sectional view showing a thin-film
piezoelectric resonator according to a fifth embodiment of the
invention.
FIG. 10A is a plan view showing a thin-film piezoelectric resonator
in the related art; and
FIG. 10B is a sectional view taken along the line A-A' in FIG.
10A.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A specific configuration of the invention will be described below
in detail.
FIGS. 1A to 1C are schematic views showing a thin-film
piezoelectric resonator according to a first embodiment of the
invention. FIG. 1A is a plan view of the thin-film piezoelectric
resonator. FIGS. 1B and 1C are sectional views taken along the line
A-A' and the line B-B' in FIG. 1A.
The resonator according to this embodiment includes a substrate 11
having a vibration space S, and an upper barrier layer 12 and a
lower barrier layer 13 provided on upper and lower surfaces of the
substrate 11 respectively. The resonator further includes a lower
electrode 14 provided on the upper barrier layer 12, a
piezoelectric thin film 15 provided on the lower electrode 14, and
an upper electrode 16 and ground electrodes 17 provided on the
piezoelectric thin film 15. The lower electrode is electrically
connected to a signal electrode through a through-hole 15a provided
in the piezoelectric thin film to thereby form a resonator of a
coplanar structure.
The upper electrode 16 and the lower electrode 14 have portions for
resonating the upper electrode 16 and the lower electrode 14
(hereinafter referred to as resonant portions 16a and 14a
respectively), and portions of the upper and lower electrodes 16
and 14 except the resonant portions 16a and 14a (hereinafter
referred to as lead-out portions 16b and 14b respectively).
Configuration is made so that the electrode thickness of at least
one part of the lead-out portion 16b or 14b in at least one of the
upper and lower electrodes 16 and 14 is larger than the electrode
thickness of the resonant portion 16a or 14a formed to be continued
from the lead-out portion. The term "formed to be continued" used
herein means that the resonant portion and the lead-out portion are
formed on the same plane and electrically connected to each other.
That is, in FIGS. 1A and 1B, for example, the lead-out portion 16b
and the resonant portion 16a in the upper electrode are formed
continuously, and the lead-out portion 14b and the resonant portion
14a in the lower electrode are formed continuously.
In the coplanar structure in which the ground electrodes are
provided, configuration may be made so that the electrode thickness
of at least part of the ground electrodes is larger than the
electrode thickness of the resonant portion of the upper or lower
electrode formed on the same plane with the ground electrodes as
shown in FIG. 1C.
Although FIGS. 1A and 1B show the case where the electrode
thickness of all the lead-out portion 16b of the upper electrode 16
is made large, part of the lead-out portion 16b may be thickened.
Alternatively, all or part of the ground electrodes 17 may be
thickened. Or the electrode thickness of at least one of the upper
electrode 16, the lower electrode 14 and the ground electrodes 17
may be made large. When part of the lead-out portion or the ground
electrodes is thickened, for example, configuration may be made so
that a neighborhood of an end portion of the lead-out portion is
thickened so as to be provided as a pad for connecting an
electrically connecting wire or the like.
Generally, the electrode thickness of each resonant portion is
limited in order to obtain a required resonant frequency. Each of
the lead-out portions and the ground electrodes, however, can have
an arbitrary thickness because the lead-out portions and the ground
electrodes have no relation to resonance. Accordingly, in order to
reduce insertion loss affected by electric resistance of the
electrodes, the electrode thickness of each of the lead-out
portions of the upper and lower electrodes and the ground
electrodes may be formed so as to be as large as possible to reduce
electric resistance of these portions to thereby reduce insertion
loss of the resonator.
The substrate 11 has a cavity S provided correspondingly to a
region in which the resonator T is disposed. The cavity S is shaped
like a rectangle in top view. For example, when an Si substrate is
used as the substrate 11, the thickness of the substrate 11 is in a
range of from about 100 .mu.m to about 1000 .mu.m.
The upper barrier layer 12 is an electrically insulating layer for
separating the substrate 11 and the lower electrode 14 from each
other so that the lower electrode 14 can be disposed in a region
corresponding to the cavity S of the substrate 11. For example,
silicon nitride (SiN.sub.x), SiO.sub.2 or the like may be used as
the material of the upper barrier layer 12. The thickness of the
upper barrier layer 12 is in a range of from about 0.03 .mu.m to
about 0.5 .mu.m.
The lower barrier layer 13 is formed as a predetermined pattern for
providing the cavity S. For example, silicon nitride (SiN.sub.x),
SiO.sub.2 or the like may be used as the material of the lower
barrier layer 13. The thickness of the lower barrier layer 13 is in
a range of from about 0.03 .mu.m to about 0.5 .mu.m.
The piezoelectric thin film 15 is a thin film having piezoelectric
characteristic. For example, zinc oxide (ZnO), lead zirconate
titanate (Pb(Zr,Ti)O.sub.3) (hereinafter referred to as PZT),
aluminum nitride (AlN) or the like may be used as the material of
the piezoelectric thin film 15. The thickness of the piezoelectric
thin film 15 is preferably selected to be not larger than 5 .mu.m.
This is because the operating frequency is lowered if the
piezoelectric thin film 15 is too thick. Accordingly, it is
preferable that the piezoelectric thin film 15 is formed to be as
thin as possible so that the operating frequency shifts to the
higher-frequency side. In consideration of the problem on
fabrication, reliability, etc., the lower limit of the thickness is
set at about 0.05 .mu.m.
According to the material used for the piezoelectric thin film 15,
for example, when PZT is used as the material of the piezoelectric
thin film 15, a buffer layer (not shown) may be provided between
the upper barrier layer and the lower electrode layer. For example,
SiO.sub.2 /TiO.sub.2, ZrO.sub.2 or the like may be used as the
material of the buffer layer. The thickness of the buffer layer may
be selected to be in a range of from about 0.03 .mu.m to about 0.5
.mu.m.
A metal such as Al, Pt, Au, Ag, Cr, Cu, Ti, etc. may be used as the
material of each of the lower and upper electrodes 14 and 16.
Before each electrode is formed, an adhesive layer may be formed on
the piezoelectric thin film from the point of view of improving
adhesion between the electrode material and the piezoelectric thin
film. For example, a chromium (Cr) layer may be formed as an
adhesive layer and a gold (Au) layer may be laminated on the whole
surface of the chromium layer so as to form a two-layered
structure. In each of the lower and upper electrodes, the thickness
of at least part of the lead-out portion is larger than the
thickness of the resonant portion. The thickness of the resonant
portion is decided on the frequency of the resonator and is
generally in a range of from about 0.03 .mu.m to about 1 .mu.m. On
the other hand, when the thickness of at least part of the lead-out
portion is made large, the upper limit of the thickness is set at
about 5 .mu.m.
When the ground electrodes 17 are provided, for example, each
ground electrode 17 is shaped like a rectangle long in one
direction on the upper surface of the piezoelectric thin film but
the shape of each ground electrode 17 is not limited thereto. When
the ground electrodes 17 are provided so that at least part of the
ground electrodes 17 is thicker than each resonant portion, the
upper limit of the thickness of each ground electrode 17 is set at
about 5 .mu.m.
In FIGS. 1A and 1B, the resonant portion 14a provided in the
neighborhood of the left end portion of the lower electrode 14 and
the resonant portion 16a provided in the neighborhood of the right
end portion of the upper electrode 16 are disposed opposite to each
other through the piezoelectric thin film 15. These resonant
portions 14a and 16a and the piezoelectric thin film 15 form the
resonator T. Although FIG. 1A shows the case where the resonant
portion 14a of the lower electrode 14 is slightly different in size
from the resonant portion 16a of the upper electrode 16 for the
sake of convenience, the size of the resonant portion 14a is
practically equal to the size of the resonant portion 16a.
The piezoelectric thin film 15 has a through-hole 15a provided in a
position corresponding to the neighborhood of an end of the lower
electrode 14 opposite to the end portion where the resonant portion
14a is provided. The lower electrode 14 is connected to a signal
conductor portion (not shown) by a bonding wire or the like (not
shown) passing through the through-hole 15a.
FIGS. 2A and 2B are schematic views showing a filter according to a
second embodiment of the invention. FIG. 2A is a plan view of the
filter. FIG. 2B is a sectional view taken along the line A-A' in
FIG. 2A. The filter is formed as a ladder filter in which
resonators are connected in series and in parallel as shown in FIG.
3.
The basic layer structure or the like is the same as in the first
embodiment shown in FIGS. 1A and 1B. Although the description of
duplicated parts will be omitted here, the structure of electrodes
as a different structure will be described below.
Lower electrodes 24A and 24B are disposed left and right in FIGS.
2A and 2B. A lower electrode 24C is disposed below the lower
electrodes 24A and 24B in FIG. 2A. Each of the lower electrodes 24A
and 24B is substantially shaped like a rectangle in plan view so
that the width of a neighborhood of an end portion of the lower
electrode adjacent to an end portion of the other lower electrode
is slightly larger than the width of the other portion. The lower
electrode 24C is shaped like a rectangle long in one direction in
plan view.
An upper electrode 26 is substantially shaped like a T figure in
plan view. The upper electrode 26 has three resonant portions 26a,
26b and 26c in the neighborhoods of end portions of the T figure,
and a lead-out portion 26d. The resonant portions 26a and 26b are
slightly wider than the other portions.
The lower electrode 24A serves also as an input terminal of the
filter. The lower electrode 24B serves also as an output terminal
of the filter. The lower electrode 24C is grounded. The lower
electrodes 24A, 24B and 24C have resonant portions 24Aa, 24Ba and
24Ca, and lead-out portions 24Ab, 24Bb, 24Cb and 24Cc
respectively.
The filter according to this embodiment is formed so that the
electrode thickness of at least part of the lead-out portion 26d,
24Ab, 24Bb, 24Cb or 24Cc in each of the upper electrode 26 and the
lower electrodes 24A, 24B and 24C is larger than the electrode
thickness of the resonant portion 26a, 26b, 26c, 24Aa, 24Ba or 24Ca
formed to be continued from the lead-out portion.
Although FIGS. 2A and 2B show the case where the whole surface of
the lead-out portion 26d of the upper electrode 26 is formed to be
thicker than the resonant portions 26a, 26b and 26c of the upper
electrode 26, part of the lead-out portion 26d may be formed to be
thicker. All or part of at least one of the lead-out portions of
the lower electrodes 24A, 24B and 24C may be thickened. At least
one of the upper electrode 26 and the lower electrodes 24A, 24B and
24C may be thickened.
The resonant portion 24Aa provided in a neighborhood of the right
end portion of the lower electrode 24A and the resonant portion 26a
provided in a neighborhood of the left end portion of the upper
electrode 26 are disposed opposite to each other through a
piezoelectric thin film 25. These resonant portions 24Aa and 26a of
the electrodes 24A and 26 and the piezoelectric thin film 25 form a
series resonator T1 on the input terminal side of the filter.
The resonant portion 24Ba provided in a neighborhood of the left
end portion of the lower electrode 24B and the resonant portion 26b
provided in a neighborhood of the right end portion of the upper
electrode 26 are disposed opposite to each other through the
piezoelectric thin film 25. These resonant portions 24Ba and 26b of
the electrodes 24B and 26 and the piezoelectric thin film 25 form a
series resonator T2 on the output terminal side of the filter.
Although FIG. 2A shows the case where the resonant portions 24Aa
and 24Ba of the lower electrodes 24A and 24B are different in size
from the resonant portions 26a and 26b of the upper electrode 26
for the sake of convenience, the resonant portions 24Aa and 24Ba
are practically equal in size to the resonant portions 26a and
26b.
The resonant portion 24Ca provided in a neighborhood of the center
of the lower electrode 24C and the resonant portion 26c provided in
a neighborhood of the lower end portion of the upper electrode 26
are disposed opposite to each other through the piezoelectric thin
film 25. These resonant portions 24Ca and 26c of the electrodes 24C
and 26 and the piezoelectric thin film 25 form a parallel resonator
T3 in the filter.
The piezoelectric thin film 25 has through-holes 25a, 25b, 25c and
25d provided in positions corresponding to the neighborhoods of end
portions of the lower electrodes 24A and 24B opposite to the end
portions where the resonant portions 24Aa and 24Ba are provided and
in positions corresponding to the neighborhoods of opposite end
portions of the lower electrode 24C, respectively. The lower
electrodes 24A and 24B are connected to signal conductor portions
(not shown) by bonding wires or the like (not shown) passing
through the through-holes 25a and 25b respectively. The lower
electrode 24C is connected to a ground conductor portion (not
shown) by bonding wires or the like (not shown) passing through the
through-holes 25c and 25d.
Though not shown, the resonators according to this embodiment may
be formed to function as a duplexer.
A method for fabricating the filter according to this embodiment
will be described below with reference to FIGS. 2A and 2B.
Incidentally, the resonator according to the embodiment shown in
FIGS. 1A and 1B may be basically fabricated by the same fabricating
method except difference in electrode structure.
First, silicon nitride (SiN.sub.x) with a predetermined thickness
is deposited by a chemical vapor deposition method on each of upper
and lower surfaces of a bare Si wafer serving as the substrate 21.
SiN.sub.x formed on the upper surface of the Si wafer forms the
upper barrier layer 22. SiN.sub.x formed on the lower surface of
the Si wafer is processed into a predetermined pattern by
photolithography and reactive ion-etching to thereby form the lower
barrier layer 23.
The lower electrodes 24A, 24B and 24C are formed by a so-called
lift-off process according to the following procedure.
First, a predetermined pattern of photo resist is formed by
photolithography. Then, metals as electrode materials, e.g.
chromium and gold (Cu/Au) with predetermined thicknesses are
deposited by sputtering. For example, a 10 nm-thick chromium layer
and a 100 nm-thick gold layer are formed. Incidentally, Cr is used
as an adhesive layer.
Then, the patterned photo resist and Cu/Au formed thereon are
removed with a solvent such as acetone to thereby obtain the lower
electrodes 24A, 24B and 24C.
Then, a piezoelectric material for forming the piezoelectric thin
film 25, e.g., zinc oxide (ZnO) with a predetermined thickness is
formed by sputtering. The piezoelectric thin film 25 is etched with
acetic acid to form through-holes 25a, 25b, 25c and 25d for
connecting bonding wires to the lower electrodes 24A, 24B and 24C
respectively.
Then, the upper electrode 26 is formed by a lift-off process. For
example, after a mask 41 shown in FIG. 4 is used so that the
electrode is formed by sputtering Cr/Au, a mask 42 shown in FIG. 4
is used so that a portion of overlap between the masks 41 and 42 in
the electrode can be thickened by sputtering Cr/Au. In this manner,
the shapes of masks can be selected so that a predetermined portion
of the electrode is thickened.
Incidentally, when a thick connection portion 26d is to be formed
in the upper electrode 26, it is generally necessary to repeat the
lift-off process twice, so that there is a problem that the number
of fabricating steps increases. For example, in a
series-parallel-series type filter (ladder filter) as shown in FIG.
3, such a problem will be insignificant particularly. That is, when
the filter is to be formed, the frequency of each series resonator
needs to differ from the frequency of the parallel resonator. When,
for example, the electrode thicknesses of the resonant portions of
the series and parallel resonators are made different to obtain
different frequencies, the filter is however formed originally on
the assumption that the lift-off process is repeated twice.
Accordingly, in the ladder filter, when masks as shown in FIG. 4
are used, the electrodes can be partially thickened to reduce
insertion loss without increase in number of steps.
Finally, the Si wafer is etched with a KOH solution from the lower
barrier layer on the back. Thus, the filter is fabricated
completely.
Although the description has been made on the case where the upper
electrode is thickened, the lower electrodes may be thickened in
the same electrode fabricating procedure as described above or the
upper and lower electrodes may be thickened.
FIGS. 5A and 5B are schematic views showing a thin-film
piezoelectric resonator according to a third embodiment of the
invention. FIG. 5A is a plan view of the thin-film piezoelectric
resonator. FIG. 5B is a sectional view taken along the line A-A' in
FIG. 5A.
The basic layer structure or the like is the same as in the first
embodiment shown in FIGS. 1A and 1B. The description of duplicated
parts will be omitted. Other parts than an upper electrode, a lower
electrode and ground electrodes are not shown in FIGS. 5A and 5B.
The structure of electrodes as a different structure will be
described below.
In the resonator according to this embodiment, at least one of the
upper electrode 34 and the lower electrode 36 is formed so that the
electrode material of the resonant portion 34a (36a) is different
from the electrode material of the lead-out portion 34b (36b)
formed to be continued from the resonant portion 34a (36a).
The resonant portion and the lead-out portion are connected to each
other so as to partially overlap each other in the connection
portion. That is, at least part of the lead-out portion 34b is
formed by stacking an electrode material different from that of the
resonant portion 34a, and one electrode of the stacked materials is
continued from the resonant portion 34a. This is for the necessity
of ensuring electric connection because insertion loss increases if
electric connection in the connection portion is insufficient.
In a coplanar structure in which the ground electrodes are
provided, configuration may be made so that the electrode material
of the ground electrodes 37 is made different from the electrode
material of the resonant portion 34a or 36a of the upper or lower
electrode 34 or 36.
A material which is low both in Poisson's ratio and in density is
preferably used as the electrode material of the resonant portions
34a and 36a. A material which is low in specific resistance is
preferably used as the electrode material of the lead-out portions
34b and 36b or the ground electrodes 37. Specifically, aluminum
(Al) is preferably used as the electrode material of the resonant
portions whereas copper (Cu) or silver (Ag) is preferably used as
the electrode material of the lead-out portions.
When a material which is low both in Poisson's ratio and in density
is used as the electrode material of the resonant portions, ripples
due to a thickness shear mode on the pass signal in the passband
can be reduced. When a material which is low in specific resistance
is used as the electrode material of the lead-out portions or the
ground electrodes, insertion loss can be reduced.
Table 1 shows characteristics of electrode materials. FIGS. 6A to
6C show impedance-frequency characteristics of electrode
materials.
TABLE 1 Specific Metal Poisson's Density Resistance Material Ratio
(10.sup.3 kg .multidot. m.sup.-3) (10.sup.-8 .OMEGA. .multidot. m)
Al 0.345 2.70 2.50 Au 0.440 19.32 2.05 Pt 0.377 21.45 9.81 Cu 0.343
6.24 1.55 Ag 0.367 10.50 1.47
The impedance-frequency characteristic shown in FIGS. 6A to 6C is
measured for different electrode materials, in a thin-film
piezoelectric resonator having a 0.1 .mu.m-thick upper barrier
layer (SiN.sub.x), a 0.1 .mu.m-thick lower electrode, a 1
.mu.m-thick piezoelectric thin film (ZnO), and a 0.1 .mu.m-thick
upper electrode.
It is obvious from FIGS. 6A to 6C that ripples are minimized when
Al which is low both in Poisson's ratio and in density is used as
the electrode material.
Although the electrode thickness in each of the lower and upper
electrodes need not be between the lead-out portion and the
resonant portion formed to be continued from the lead-out portion,
configuration may be preferably made so that the electrode
thickness of at least part of the lead-out portion is larger than
the electrode thickness of the resonant portion formed to be
continued from the lead-out portion in the same manner as in the
first embodiment of the invention. In a coplanar structure in which
the ground electrodes are provided, configuration may be preferably
made so that the electrode thickness of at least part of the ground
electrodes is larger than the electrode thickness of the resonant
portion of the upper or lower electrode formed on the same plane
with the ground electrodes.
In the ladder filter as shown in FIG. 3, configuration may be made
so that the electrode material of each resonant portion in at least
one of the lower and upper electrodes is different from the
electrode material of the lead-out portion formed to be continued
from the resonant portion.
Though not shown, resonators according to this embodiment may be
formed to function as a duplexer.
A method for producing the resonator according to the third
embodiment of the invention will be described below with reference
to FIGS. 5A and 5B.
First, silicon nitride (SiN.sub.x) with a predetermined thickness
is deposited by a chemical vapor deposition method on each of upper
and lower surfaces of a bare Si wafer serving as the substrate.
SiN.sub.x formed on the upper surface of the Si wafer forms the
upper barrier layer. SiN.sub.x formed on the lower surface of the
Si wafer is processed into a predetermined pattern by
photolithography and reactive ion-etching to thereby form the lower
barrier layer. (The layer structure is the same as in FIGS. 1A and
1B).
The lower electrode 36 is formed by a so-called lift-off process
according to the following procedure.
First, a predetermined pattern of photo resist is formed by
photolithography. Then, metals as electrode materials, e.g.,
chromium and gold (Cu/Au) with a thickness of 10 nm and a thickness
of 100 nm respectively are deposited by sputtering. Incidentally,
Cr is used as an adhesive layer.
Then, the patterned photo resist and Cu/Au formed thereon are
removed with a solvent such as acetone to thereby obtain the lower
electrode 36.
Then, a piezoelectric material for forming the piezoelectric thin
film 35, e.g., zinc oxide (ZnO) with a predetermined thickness is
formed by sputtering. The piezoelectric thin film is etched with
acetic acid to form a through-hole 35a for connecting a bonding
wire to the lower electrode 36.
Then, the upper electrode 34 is formed by a lift-off process. For
example, after a mask 43 shown in FIG. 7 is used so that the
resonant portion 34a of the electrode is formed by sputtering Al, a
mask 44 shown in FIG. 7 is used so that the lead-out portion 34b of
the electrode is formed by sputtering Cr/Au. When the electrode
material of the lower electrode is changed, the lift-off process
may be repeated twice in the same manner as described above while
the masks of predetermined shapes are used.
Finally, the Si wafer is etched with a KOH solution from the lower
barrier layer on the back. Thus, the resonator is fabricated
completely.
FIG. 8 is a schematic sectional view showing a thin-film
piezoelectric resonator according to a fourth embodiment of the
invention.
The basic layer structure or the like is the same as in the first
embodiment shown in FIGS. 1A and 1B except that the vibration space
is replaced by a cavity 48 provided between a lower barrier layer
42 and a substrate 41.
Incidentally, upper and lower electrodes 46 and 44 are provided on
upper and lower surfaces of a piezoelectric thin film 45
respectively and configuration is made so that a lead-out portion
of the upper electrode is thicker than a resonant portion of the
upper electrode, in the same manner as in FIGS. 1A and 1B.
In at least one of the upper and lower electrodes, configuration
may be made so that the electrode material of the resonant portion
is different from the electrode material of the lead-out portion
formed to be continued from the resonant portion, in the same
manner as in the third embodiment.
FIG. 9 is a schematic sectional view showing a thin-film
piezoelectric resonator according to a fifth embodiment of the
invention.
The basic layer structure or the like is the same as in the first
embodiment shown in FIGS. 1A and 1B except that the vibration space
is replaced by an acoustic multi-layer film of layers 58a and 58b
provided between a lower electrode 54 and a substrate 51. The
acoustic multi-layer film is formed by lamination of layers of a
high acoustic impedance material such as aluminum nitride and
layers of a low acoustic impedance material such as silicon oxide.
Although FIG. 9 shows the case where the acoustic multi-layer film
is composed of two layers 58a and 58b, the acoustic multi-layer
film may be composed of three or more layers so that layers of a
high acoustic impedance material and layers of a low acoustic
impedance material are laminated alternately.
Incidentally, upper and lower electrodes 56 and 54 are provided on
upper and lower surfaces of a piezoelectric thin film 55
respectively and configuration is made so that a lead-out portion
of the upper electrode is thicker than a resonant portion of the
upper electrode, in the same manner as in FIGS. 1A and 1B.
In at least one of the upper and lower electrodes, configuration
may be made so that the electrode material of the resonant portion
is different from the electrode material of the lead-out portion
formed to be continued from the resonant portion, in the same
manner as in the third embodiment.
In the thin-film piezoelectric resonator, the filter, the duplexer
and the method for fabricating the thin-film piezoelectric
resonator according to the invention, insertion loss can be
reduced. In addition, ripples can be reduced.
* * * * *